How to solve an XY-Wing in Sudoku
An XY-Wing is a pivot cell with two candidates and two wing cells, which together force a digit out of the cells the wings both see.
How it works
The pivot holds X and Y. One wing, which the pivot sees, holds X and Z. The other wing, also seen by the pivot, holds Y and Z.
Whichever way the pivot goes, one of the wings must become Z: if the pivot is X, the first wing is Z; if it is Y, the second is. So any cell that sees both wings cannot be Z.
How to spot it
Start with a cell that has two candidates and find two other two-candidate cells that share one digit each with it and the same third digit with each other.
A worked example
This is a step from a real puzzle, a moment where the next move needs an XY-Wing.
P is the pivot, W are its two wings. Struck-through digits are the ones that can be removed.
R4C2 (2/5) sees R5C1 (2/3) and R4C7 (5/3). Whichever way R4C2 goes, one of the other two must be 3, so remove 3 from cells that see both: R5C7.
Where it fits in Rocksled Sudoku
Puzzles that need an XY-Wing are never rated below Expert. Each step of this kind adds 62 points to a puzzle’s difficulty score. When you ask for a hint that needs it, the game names the technique and shows the cells involved. See how to play for the rest of the controls.